1. Field of the Invention
The present invention relates to an interrogator that is to be incorporated in a communication system in which each of at least one transponder is operable, upon reception of a main carrier wave transmitted from the interrogator, to respond to the interrogator with a reflected wave that is generated by modulating the main carrier wave.
2. Discussion of Related Art
There is known a communication system in which each of at least one transponder is operable, upon reception of a main carrier wave transmitted from the interrogator, to respond to the interrogator with a reflected wave that is generated by modulating the main carrier wave. For example, JP-3105825 B2 (publication of Japanese Patent issued in 2000) discloses an automatic gain controller circuit which is arranged to accurately detect a signal related to modulation performed by the transponder and which is accordingly applicable to such a communication system. This automatic gain controller circuit includes two circuits, wherein a first level detection/determination circuit as one of the two circuits is operated, where an actual electric field for input reception is considerably deviated from a desired electric field due to movement of the transponder, interference wave or other influencing factor, to adjust a gain corresponding to the deviation, and a second level detection/determination circuit as the other of the two circuits is operated, after the operation of the first level detection/determination circuit, to finely adjust the gain. That is, in the automatic gain controller circuit, realization of an accurate and rapid control of the gain is intended.
However, since the signal related to the modulation performed by the transponder, i.e., transmitted information signal is provided by an extremely small signal, the conventional technique does not necessarily make it possible to satisfactorily detect the signal related to the modulation performed by the transponder, for example, where a communication distance is relatively long or where there exist reflected waves respectively transmitted from a plurality of transponders.